Different methods are known for the deposition of solid particulate impurities from air or liquids. The simplest methods are based on the deposition of the particles on filter membranes both in gaseous and also liquid media with subsequent analysis by means of suitable methods such as light microscopy, scanning electron microscopy or gravimetric analysis (see for example Millipore Particle Monitoring Guide, Millipore Corporation, 1998).
The filter membranes generally comprise polymeric materials, such as for example nitrocellulose, nylon, FTFE or PVC, of an exactly defined pore size, wherein the particles of larger diameter than the pore width of the filter accumulate on the latter and can subsequently be analysed. In recent times, for many applications in the area of microelectronics, ascertaining and analysing particularly small particles, so-called micro-particles, in the range of sizes of about 10 μm or less, is of particular interest, the analysis of which is problematical with the hitherto known methods, by virtue of the size relationships of the particles to be analyzed.
Metal filters are also known, such as for example metal filters consisting of silver, for filtration purposes, from Millipore, which however because of their method of manufacture have a surface which, by virtue of its roughness, is not suitable for the recognition or identification of individual particles of <5 μm. The corporation alto tec GmbH of Hamburg offers gold-plated filters for determining asbestos concentrations, which are also not optimized for the described use.
Procedures exist for the quantitative contamination analysis of smooth surfaces, such procedures using a laser beam and a laser scanner for scanning surfaces and detecting deviations from a plane by means of the scattered light which is collected with a photodetector. Such a method is set forth in U.S. Pat. No. 5,479,252. It is however not possible to implement chemical characterization of the particles with that method.
Other methods, such as that set forth, for example, in U.S. Pat. No. 6,178,383, investigate video images in digital form with image recognition programs and, besides the recognition of particulate impurities, can also provide information about the shape and/or size thereof. The equipment for methods of that kind however is very costly in comparison with the laser technology, and also identification of the particles with that method is not possible. The resolution of those methods is admittedly theoretically only diffraction-limited but it is difficult to determine the size of particles which are smaller than 1.5 μm.
Methods of Raman spectroscopy are known for the qualitative and quantitative analysis of the composition of a sample, in particular of microparticles (M Lankers, J Popp, G Rössling and W Kiefer, Chem Phys Let 277 (1997) 331–334) and have proven to be advantageous. In that case, a sample is irradiated with intensive electromagnetic monochromatic radiation, for example laser light. For that purpose, electromagnetic radiation from the visible or ultraviolet spectral range is usually employed. Upon measurement of the scattered light with a spectrometer and a suitable detector, that is to say when determining the beam intensity of the scattered light as a function of wavelength, the result obtained is a spectrum which comprises a strong line, the so-called exciter line, and very many weaker lines, the so-called Raman lines. The exciter line has the same wave number as the incident radiation. The Raman lines respectively correspond to specific rotational or vibrational states of the substance to be investigated. The Raman lines are arranged on a wave number scale symmetrically with respect to the exciter line. In addition the Raman lines are of an intensity which is between 10−3 and 10−4 times less, with the intensity of the Raman lines on the low-frequency side usually being substantially greater at ambient temperature than those on the higher-frequency side.
The Raman spectrum, that is to say the sequences of Raman lines, is characteristic in respect of each substance. A compound can be identified by comparison of its spectrum with the spectra of known compounds.
It will be noted however that the low level of efficiency of Raman spectroscopy is found to be problematical when using that procedure. It is necessary to use very high laser powers for investigating small amounts of substances, as is the case when investigating microparticles. In that respect it is undesirable that the focus of the laser beam is generally markedly larger than the diameter of the particle. Thus there is the unwanted consequence that the signal of the supporting substrate is recorded at the same time and in that situation the spectrum of the particle is slightly overlapped. That becomes clear from the area relationships. If a focus of about 10 μm in diameter is used in order to investigate a particle of a diameter of 1 μm, the supporting substrate/particle signal relationship is about 10:1. In most cases that makes it impossible to characterize the particle. In some cases it is possible to resolve the problem by focusing the laser beam to 1 μm. In that situation however the energy density rises severely and results in damage or a modification as a consequence of burning or photochemical reactions on the part of sensitive substances.
Therefore, certain embodiments of the present invention provide supporting substrates for the spectroscopic analysis of particles, preferably Raman spectroscopy, which reduce the above-indicated disadvantages, in particular in the analysis of microparticles, to such a degree that reliable analysis results are obtained and which in addition are suitable for the filtration of both liquid and also gaseous media.
Further limitations and disadvantages of conventional, traditional, and proposed approaches will become apparent to one of skill in the art, through comparison of such systems and methods with the present invention as set forth in the remainder of the present application with reference to the drawings.